Method and device for specific absorption rate measurement of an electric field

ABSTRACT

A method for calculating the Specific Absorption Rate (SAR) caused in a body by the electric field of a wireless communications device, and comprises using a model of said body. The device is placed in the proximity of the model, and the electric field is measured at discrete points. The method comprises measuring the magnitude of the electric field in points of a first and a second surface in the model. By means of the measured magnitudes, the phase in said points is determined, so that the complex electric field in said points is determined, and the complex electric field in said points is used to determine the complex electric field in said model. The complex electric field is used in order to calculate the SAR value caused by the device.

TECHNICAL FIELD

The present invention relates to the measuring of the SpecificAbsorption Rate (SAR) of the electric field emitted by a wirelesscommunications device.

By means of the invention, such measurements can be performed morerapidly and with greater ease than previously.

BACKGROUND

Dosimetric Specific Absorption Rate (SAR) measurements for, inter alia,mobile phone and radio base station antennas are widely used today.

In the standard procedure used today, a probe is used to scan a volumeinside a “phantom”, i.e. a model intended to resemble the human body,usually a container filled with a body-tissue equivalent liquid.

The probe is used to register the amplitude of the vector components ofthe electric field emitted by the device which is to be measured. Theantenna or device under test is placed on or near the surface of thephantom. The amplitude of the electric field vector components aremeasured, and the mass-averaged SAR values are determined, for exampleby means of a sliding spatial averaging.

Examples of existing solutions for SAR measurements are volumetric scanin the entire volume, sparse volumetric scan with data fitting to agiven model, and area scan of SAR and propagation into a volume usingattenuation factors from previous experiments.

The existing solutions provide good SAR measurements, but can be said tohave drawbacks in that they are relatively time-consuming, and in somecases model dependent, i.e. tailored to a certain device or type ofdevice.

Patent EP 1 615 041 discloses a device for measuring the SAR value of acellular telephone, but the device of that document measures both theamplitude and the phase of an electric or magnetic field.

Patent U.S. Pat. No. 5,789,929 discloses a method for SAR measurementswhich involves measuring two orthogonal magnetic fields, and does notappear to disclose anything regarding the use of measured amplitudes ofthe magnetic or the electric field in order to arrive at the complexelectric field.

SUMMARY

As disclosed above, there is thus a need for a method which can be usedto carry out SAR measurements more rapidly than present solutions, andwhich is also independent of the model or kind of device which is to bemeasured.

This need is addressed by the present invention in that it discloses amethod for calculating the Specific Absorption Rate (SAR) in a body ofthe electric field emitted by a wireless communications device.

The method comprises the use of a model of the body in which the SARvalue is to be calculated. The device which is to be measured is placedin the proximity of said model, and the electric field components aremeasured at specific points in the model.

According to the method of the invention, the measurements of theelectric field components are carried out in points of a first and asecond surface in the model, and the measurements comprise the magnitudeof the electrical field components in said points.

The measured magnitudes are used in order to determine the phase of theelectric field components in said points, so that the complex electricfield in said points is determined.

Additionally, the complex electric field in said points is used in orderto determine the complex electric field over an entire volume ofinterest in the model, and the complex electric field over said entirevolume is used in order to calculate the SAR value caused by theelectric field emitted by the device over the volume of interest.

Since the method of the invention can be carried out by means ofmeasurements in two surfaces in the body, as opposed to previously knownmethods, it will be realized that significant advantages can be gainedusing the invention with regard to the time that is necessary forobtaining the SAR value.

Additional advantages are obtained since the method of the invention canbe used to determine the SAR value using only measurements of themagnitude of the field in the measurement points, as opposed to methodswhich measure the complex field in the measurements points, i.e. bothamplitude and phase.

The term “volume of interest” is used in this text in order illustratethat only parts of the entire volume of the body or model of the bodymay be of interest when making the measurements in question.

The term “field components” is used in order to signify the fact thatthe electric field in each measurement point comprises three orthogonalcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows a schematic overview of a measurement set-up according tothe invention, and

FIG. 2 shows a flow chart for use in the invention.

DETAILED DESCRIPTION

In FIG. 1, a measurement set-up 100 for use with the invention is shown.In the set-up 100, there has been positioned a device 110 which is to betested for electric emissions. The invention can be used for virtuallyany device which emits electric radiation, but the invention will bedescribed using a cellular telephone as the device 110. However,examples of other devices to which the invention might be applied arecordless telephones, cordless microphones, auxiliary broadcast devicesand radio parts of various sizes intended for computers.

As has been described above, the invention is directed towards measuringthe Dosimetric Specific Absorption Rate (SAR) for the device 110. TheSAR value which it is desired to determine is usually that in a humanbody which is in the proximity of the device in the question.

In order to simulate a human body, a so called “phantom” 120 is used,i.e. a model of the human body. The model 120 can be of many kinds, butin the embodiment shown in FIG. 1 a container filled with a liquid whichhas tissue equivalent electric properties.

In order to measure the electric field caused by the device 110 insidethe model 120, a vector field probe 130 is used.

In conventional SAR measurements, the probe 130 would have been movedover points in a volume of the model 120, by means of which the complexelectric field in the model 120 would have been determined. This is amethod which works well, but which is inherently time-consuming,something which will be particularly bothersome in whole-body SARmeasurements.

In the method of the invention, the device 110 to be measured is, insimilarity to earlier methods, placed in the proximity of the model 120,and the electric field is measured at specific points in the model 120.However, in the method of the invention, the measurements of theelectric field are only carried out in points of a number of surfaces inthe model 120.

For reference, a grid 140 has been superimposed on the test set-up inFIG. 1.

In a preferred embodiment, as shown in FIG. 1, the measurements arecarried out in a first 150 and a second 160 surface. As shown in FIG. 1,in a preferred embodiment the measurement surfaces are planar, but thesurfaces can also be curved, the method of the invention works equallywell for both types of surfaces.

As also shown in FIG. 1, the measurement surfaces 150, 160, are parallelto one another in a preferred embodiment, but in other embodiments thesurfaces need not be parallel.

It will be realized that since the method of the invention only measuresthe electric field in two surfaces as opposed to measurements carriedout over the entire volume of the model 120, significant benefits areobtained by means of the invention regarding the time needed for themeasurements.

In order to further minimize the time needed for the actualmeasurements, the measurements comprise measuring only the magnitude(amplitude) of the electric field in said points.

The measured amplitudes are then used in order to determine the phase ofthe electric field in said points, so that the complex electric field insaid points is determined. The exact mechanism for doing this will beelaborated upon in more detail in the following. The term “complexelectric field” is here used to signify the fact that the electric fieldcomprises both amplitude and phase.

With the aid of the complex electric field in the measurement points,the complex electric field is determined over an entire volume ofinterest in said model and by means of the complex electric field overthe entire volume, the SAR caused by the device 110 over said volume iscalculated.

The mechanism by which the complex electric is determined in themeasurement points in the first 150 and second 160 surfaces using themeasured amplitudes will now be described in more detail with referenceto the flow chart of FIG. 2.

However, before the flow chart 200 is commented upon here, it will bepointed out that the propagation of the complex electric field betweenthe first and second measurement surfaces can be descried by means of atransfer function T, and the propagation in the other direction can thusbe described using the inverse of said transfer function, referred to inthe following as T⁻¹.

The transfer function as such is well known to those in the field, andaccordingly, if the complex electric field at one of the first andsecond surfaces is known, the complex electric field at the othersurface will also be known.

The algorithm 200 of FIG. 2 can briefly be described as follows: themeasured amplitudes at one of the surfaces, for example the firstsurface, are used, and the phase at said points is assumed to be acertain value, for example zero.

The complex electric field with those values, i.e. the measuredamplitude and the assumed phase, is passed through the transfer functionT, which describes the propagation from the first surface to the secondsurface. The amplitudes which are arrived at then are compared to themeasured amplitudes at the second surface, and if the discrepancy isbelow a certain threshold, the assumed phase value at the first surfaceis considered correct, and thus, the complex field at the first surfaceis described in full, since both the amplitude and the phase are nowknown.

The complex electric field at the second surface is also assumed to beknown, since the discrepancy between the calculated amplitudes and themeasured amplitudes at the points in the second surface is sufficientlysmall.

The phase values at the points in the second surface are also assumed tobe correct, since the discrepancy between the calculated amplitudes andthe measured amplitudes at the points in the second surface issufficiently small, i.e. below a predefined threshold.

Since the complex electric field is now known at both the first and thesecond surfaces, the transfer function T can be used in order tocalculate the complex electric field over the entire volume of interestin the body 120, and the SAR values can be calculated.

Returning now to the flowchart 200 of FIG. 2, the method of theinvention will be described in more detail.

As a first step, denoted as step 210, the measured amplitudes at thepoints in the first 150 and second 160 surfaces in the body 120 arecompiled for use.

Then, a phase value distribution is assumed for the first surface 150,which assumed phase value distribution is used in the next step of theflow chart.

The assumed phase value distribution can consist of more or lessarbitrarily chosen values, since it is only the starting point for aniterative method, but a value of zero is used in a preferred embodimentof the invention. The phase distribution value in the first surface isdenoted as φ₁ ^(c), in order to show that the phase value in each pointof the surface comprises three components, one in each direction x, y,z, of the coordinate system show in FIG. 1. The superscript “c” is thusintended to show that φ₁ comprises three orthogonal vector components,here referred to as (x, y, z).

Step 214 shows that the complex electric field E₁ in the first surfaceis assumed to consist of the measured amplitude A₁ and the assumed phaseφ₁. Again, in order to show that both the amplitude A₁ and the phase φ₁in each measurement point comprises three values, one in each directionx, y, z, of the coordinate system of FIG. 1, the field E₁ is hereexpressed as a sum over x, y, z, of A₁ and φ₁: E₁=ΣA₁ ^(c)exp(jφ₁^(c))ĉ, with the superscript c ranging over x, y, z. The meaning of thesuperscript “c” is now assumed to be familiar to the reader, and willthus not be explained again in this text.

The complex electric field E₁ which is thus assumed is then, asindicated in block 216, subjected to the transfer function T, which isthe transfer function that describes the field propagation from thefirst surface 150 to the second surface 160. Such transfer functions arewell known to those skilled in the field, and will thus not be explainedin further detail here.

Thus, in this way, an assumed (calculated) field E₂ at the secondsurface 160 is arrived at.

A check is then performed of the assumed field at the second surface160. This test, as shown in block 218, consists of comparing themeasured amplitude A₂ at the points in the second surface 160 with theassumed amplitude a₂ in the second surface.

If the discrepancy Δ₂ between the measured and the assumed amplitude inthe second surface 160 is smaller than a predetermined threshold, theprocedure is interrupted, as shown in block 219, since the field at thesecond surface 160 is deemed to be correct, and consequently, the fieldat the first surface 150 is also deemed to be correct, since the assumedphase at the first surface 150 together with the measured amplitudes atthe first field resulted in correct amplitudes at the second field.

The procedure which is followed if the electric field at the twosurfaces is deemed to be known will be outlined further down in thisdescription.

If the discrepancy Δ₂ between the measured and the assumed amplitude inthe second surface 160 exceeds the predetermined threshold, the assumedfield in the first surface as well as the calculated field in the secondsurface are deemed to be incorrect, and the procedure continues in thefollowing manner: as shown in block 220, the field E₂ in the secondsurface 160 is now assumed to comprise the measured field amplitude A₂together with the calculated or assumed phase values φ₂.

The assumed field in the second surface is then subjected to T⁻¹, i.e.the inverse function of the transformation function T. Thus, T⁻¹ is thefunction which can describe the field propagation from the secondsurface to the first surface, and accordingly, if the field at thesecond surface has been correctly assumed, the transformation T⁻¹ willresult in a correct picture of the field at the first surface.

The transformation by means of T⁻¹ can be described in mathematicalterms as: E₁=T⁻¹[E₂], which is also shown in step 222 of the flow chart200 in FIG. 2.

The field E₁ which is thus arrived at should present a correct pictureof the filed at the first surface 150 if everything is correct. In orderto check the correctness of the calculated field E₁ against the realfield at the first surface, the amplitude a₁ of the calculated field ischecked against the measured amplitude A₁ at the first surface 150.

If the discrepancy between the calculated field a₁ and the measuredfield A₁ is below a certain predetermined threshold Δ₁, the calculatedfield at the first surface is deemed to be sufficiently true.Accordingly, the field which was used to generate or calculate theaccepted field at the first surface is also deemed to be sufficientlytrue, i.e. the field at the second surface, as arrived at in step 220 isaccepted as the field at that surface, and the procedure for generatingthe complex electric fields at the first 150 and second 160 surfaces isstopped, as shown in block 226 of the flow chart 200, since it is deemedthat sufficiently good pictures of the fields E₁ and E₂ have beenobtained, as indicated in block 228.

If the discrepancy between the calculated field a₁ and the measuredfield A₁ is not below said certain predetermined threshold, theprocedure starts over again at block 214 as described above, until oneof the two fields meets the criteria described above with reference toblocks 218 and 224 in the flow chart 200 in FIG. 2. The value of φ₁ ^(c)which is used is that which was arrived at in the calculation of block222.

Suitable values for accepting the calculated amplitudes a₁, a₂, whencompared to the measured amplitudes A₁, A₂, have proven to bediscrepancies of 1% or less.

Since the electric field in at least two surfaces 150, 160, of the body120 has been determined by means of the procedure described above, thetransformation function T can now be used in order to determine thecomplex electric field in the entire body 120, or over an entire volumeof interest in that body.

The complex electric field over the entire volume of interest in thebody can then be used in order to arrive at the SAR value for the device120. This is suitably done by means of the T function used previously.

The invention is not limited to the examples of embodiments which havebeen shown above, but may be freely varied within the scope of theappended patent claims. For example, more than two surfaces may be usedin the procedure describe din the flow chart 200, and other kinds ofbodies than the human body may be simulated by means of appropriatephantoms or models.

Also, as has been pointed out above, the inventive method and apparatusmay be applied to other devices than cellular telephones, the method maybe used with more or less any kind of device with emissions that it isdesired to measure the impact of. Examples of such devices are cordlesstelephones, cordless microphones, auxiliary broadcast devices and radioparts of various sizes intended for computers.

As has been pointed out, the measurements in the two surfaces arecarried out in distinct points in the surfaces. With reference to thecoordinate system of FIG. 1, suitable distances between the points inone and the same surface have proven to be in the area of λ/2 or less,with λ being the wavelength of the measured field in the liquid used.Suitable values for the distance between the two surfaces have proven tobe in the area of λ/8 to λ/4.

1.-12. (canceled)
 13. A method for calculating the Specific AbsorptionRate (SAR) caused in a body by the electric field emitted by a wirelesscommunications device, the method comprising the steps of: using a modelof the body in which the SAR value is to be calculated; placing thedevice in question in the proximity of said model; measuring theelectric field components at specific points in said model; carrying outsaid measurements of the electric field components in points of a firstand a second surface in said model; letting said measurements comprisemeasuring the magnitude of the electric field components in said points;using the measured magnitudes in order to determine the phase of theelectric field components in said points, so that the complex electricfield in said points is determined; using the complex electric field insaid points in order to determine the complex electric field over anentire volume of interest in said model; determining the complex fieldin a first of said surfaces by using the measured field magnitude in thefirst surface, and an assumed phase value for the first surface, so thata complex field in the first surface is assumed, and the assumed complexfield in the first surface is used in order to calculate a complex fieldE₂ for the second surface; and using the complex electric field oversaid entire volume in order to calculate the SAR value caused by saiddevice over said volume.
 14. The method of claim 13, according to whichsaid first and second surfaces are straight.
 15. The method of claim 13,according to which said first and second surfaces are curved.
 16. Themethod of claim 13, according to which the surfaces are parallel to oneanother.
 17. The method of claim 13, according to which the surfaces arenon-parallel with regard to one another.
 18. The method of claim 13,according to which the complex electric field magnitude in said pointsis measured for each of the components E_(x), E_(y), E_(z) in the field,and the phase in each of said components is consequently determined. 19.A device for calculating the Specific Absorption Rate (SAR) caused in abody by the electric field emitted by a wireless communicationstransmitter, the device comprising: a model of the body in which the SARvalue is to be calculated; means for holding the device in question inthe proximity of said model; means for measuring the electric fieldcomponents at specific points in said model; means for carrying out saidmeasurements of the electric field in points of a first and a secondsurface in said model; means for letting said measurements comprisemeasuring the magnitude of the electric field components in said points;means for using the measured magnitudes in order to determine the phaseof the electric field components in said points, so that the complexelectric field in said points is determined, means for using the complexelectric field in said points in order to determine the complex electricfield over an entire volume of interest in said model; and means forusing the complex electric field over said entire volume in order tocalculate the SAR value caused by said device over said volume, whereinin such device the complex electric field in a first of said surfaces isdetermined by means of the measured field magnitude in the firstsurface, and an assumed phase value for the first surface, so that acomplex field in the first surface is assumed, and the assumed complexfield in the first surface is used in order to calculate a complex fieldfor the second surface.
 20. The device of claim 19, in which said firstand second surfaces are planar.
 21. The device of claim 20, in whichsaid first and second surfaces are curved.
 22. The device of claim 19,in which the surfaces are parallel to one another.
 23. The device ofclaim 19, in which the surfaces are non-parallel with regard to oneanother.
 24. The device of claim 19, in which the electric fieldmagnitude in said points is measured for each of the components E_(x),E_(y), E_(z) in the field, the device comprising means for consequentlydetermining the phase in each of said components.